Alarm system



March 1944- M. H. A. LINDSAY EI'AL 2,343,987

ALARM SYSTEM Filed April 19, 1940 FIG. I 1,.

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ATTORNEY whereby he can Patented Mar. 14, 1944 ALARM SYSTEM I Maxwell H. A. Lindsay, Summit, N. .Lrand Konstantin Woloscliak, New York, N. 2.,

lssignors to American District Telegraph Company. Jer- My City.

N. 1., a corporation of New Jersey Application April 19, 1940, Serial No. 330,464 Claims. (01. 177-314) This invention relates to electric signaling systems generallyand more particularly to alarm systems of the type which are sensitive to changes of electrical capacity and which are particularly adaptedto detect alarm conditions such as fires or the approach of an unlawful intruder to an object or place to be protected.

. This invention contemplates the use of what is known as a capacity alarm system for property protection and intruder detection within a subscriber's premises wherein the approach of an unlawful intruder or the occurrence of any dangerous condition disturbs the balance or some other condition within the protective circuit by means of changes in electrical capacity. The circuit then transmits a signal to an alarm device which may be located within the subscriber's premises or at a central station. In the capacity alarm system it is only necessary to provide an antenna at the place to be protected, such an antenna being easy to conceal and substantially trouble proof. Such systems thereby obviate the necessity of using contact detecting devices or other mechanical and electrical devices which are disturbed by local conditions such as heat; weather or various forms of tampering.

A further advantage of such as system is that a minimum amount of equipment is necessary. In the usual form of burglar alarm systems, a large amount of wiring must be installed, such wiring becoming useless for other purposes, whereas the equipment necessary to carry out this invention requires a minimum amount of wiring and permits numerous successive installations of a single system.

Previous systems of this character have been inherently subject to defeat in various ways and have also had the disadvantage that their sensitivity is affected by temperature and humidity conditions.

Previous capacity alarm systems have also been deficient in that no means has been provided for testing the circuit when it is put into operation whereby the subscriber for alarm service has notbeen able to ascertain whether or not his system is operative. Capacity alarm systems can be compensated to overcome small changes in weather conditions or other local conditions which might Y render the system inoperative or out of balance but the fact still remains that such systems sometimes require slight adjustment in order to obtain high quality operation. Therefore it is not only necessary to provide the central station operator with a testing means at the central station supervise the remote detecting apparatus but the unskilled subscriber himself must be provided with a simple and yet effective testing circuit whereby he may be certain that the detection system will operate, in case Of need, in the manner in which it was intended to operate.

Accordingly the principal object of this inventionis to provide an improved alarm system.

Another object of this invention is to provide an improved capacity alarm system.

Still another object of this invention is to provide an improved alarm system of the capacity type which is extremely sensitive and which cannot be defeated.

A further object of the invention is to provide a system of the capacity type which may be tested by the subscriber and by the central station vide an alarm system of the capacity type which is extremely sensitive and which may be tested by an unskilled operator with simple testing equipment.

Further objects and advantages of this invention will be obvious to those skilled in the art from an inspection ofthe accompanying speciflcation and drawing, wherein Fig. 1 is a circuit diagram of one modification of this invention, and

Fig. 2 is a circuit diagram of a further cation of this invention wherein an sensitive circuit is illustrated.

1 Referring to Fig. lot the drawing, the protected object is shown in the form of a safe or cabinet I. This protected object may take the from of any desired device or area which it is desired to protect, the safe being shown for purposes of illustration only. The protective circuit. located in a subscribers premises, is made up of a capacity and inductance bridge and a vacuum tube oscillator III which is connectedto the pmtected safe I and the bridge circuit by means of the antennae 2 and 3. This connection is shown by way of example only as it is also feasible to protect an object or area by means of a loop antenna. The bridge circuit comprises an adjustable condenser 5 in one arm thereof, an inductance 6 in another arm thereof, and inductance 'l in the third arm of the bridge. The fourth modiextremely arm of the bridge is the capacity a which is the capacity to ground of the safe I.

The grid circuitof the tube It is connected to the antenna 2 through coupling resistor II and condenser i 2 while the filament of tube III is connected to the grid circuit through resistor II.

The bridge circuit is connected to the filament of tube In by means of conductor l6 and is couplained later on in the description. The potential which appears across the coil 20 is rectified in the grid circuit of tube l8 by the grid leak 24 and condenser 25 whereby a' direct current potential biases the ,grid circuit of tube l8.

The output of tube alarm relay '23 which controls the alarm device 28 at a central oiilce by means of contact 21 and transmission line 48. A day and night" switch 45 is shown in the night position but during business hours it is thrown to the ground contact to provide test facilities. In view of changing weather conditions and changing electrical conditions in the vicinity of the detecting apparatus, a testing circuit is provided having such characteristics that it can be operated satisfactorily by the person subscribing to the protection service. Such persons are usually unskilled in the signaling art and therefore it is necessary to provide simple, yet adequate, testing circuits for indicating whether or not the circuit is in operative condition or whether it is too sensitive or not sensitive enough. Therefore, the buzzer 30 is provided in the subscriber's premises for indieating the condition of relay 23, or in other words, for indicating whether or not the alarm device 28 is operating. This indicating buzzer 30 may be used for indicating an alarm condition or for indicating lack of sensitivity, as explained later. A condenser 32 which is the equivalent of the capacity of a person approaching the protected safe or other object I is included in the test circuit. Contacts 33 of switch 34 connect the condenser 32 across the bridge circuit. Lamp 35 is provided for indicating too much sensitivity, contacts 35 of switch 34 connecting the signal lamp 36 in series with a source of alternating current 38 and the contacts 4| of relay 40 which is connected in series with the relay 26 and battery 43.

It is also important that the central station operator be enabled to test the circuit and therefore a switch 45 in. series with battery 46 establishes a circuit through the transmission line 48 to relay 50 which connects condenser 32 in the bridge circuit through contact i.

The circuit described above is intended for installation in stores or other commercial establishments which are under the supervision of persons unskilled in the art of signaling and it is important that they be enabled to determine positively whether or not their burglar protection system is operating properly. This type of burglar alarm system is normally inactive or disconnected throughout the normal business hours of the protected establishment and therefore at closing time the proprietor operates switch 34 to close contacts 33 which connect the condenser 32 to ground to simulate thereby the approach of a person to the protected object i. If the con denser 5 is properly adjusted, the bridge circuit will be normally unbalanced and there is a po-- tential difference between the grid and cathode tube Hi, this potential being 180 out of phase with respect to the potential between cathode and plate which is kept at ground potential by condenser 22. Therefore, oscillations are maintained between grid and cathode ofgthe tube H) which appear across coils 8 and I and a potential is built up across the coil 23 whereby the tube I8 is biased by its grid in such a manner that its output is substantially. zero. When condenser 32 is inserted across the bridge circuit, the bridge will become balanced and there will not be any oscillating potential between the grid and cathla is connected to a local ode of tube I0. There will be no feedbacir li'uui the plate to the grid circuit and thus the tube l 0 will no longer oscillate and the bias on tube It is reduced to zero. When the bias of tube I3 is reduced to zero the current in its plate circuit increases to such a point that relay 25 is operated. Contacts 29 of relay 26 close and energize buzzer 30 to inform the proprietor that the alarm relay has operated. During such test procedure switch 49 is in its ground connection position so that alarm device 23 is not energized.

It may be possible that the condenser 5 is not properly adjusted and therefore closure of switch contacts 33 does not produce a signal on buzzer 30, in which case condenser 5 is adjusted until an alarm condition is indicated by the buzzer. This test alone is not sufficient, however, as the circuit might be too sensitive and therefore condenser 5 must be adjusted in such a manner that the bridge becomes balanced when a person approaches the protected object within a predetermined distance and unbalanced by a certain definite amount when no person is close to the protected object I. Therefore it is necessary to test for the proper operating point of the protective circuit. Switch 34 is operated to close the contacts 35 and insert lamp 35 in circuit with the alternating current source 38 and contacts 4i of relay 43.

Relay 4B is adjusted to operate ata predetermined critical current value which if exceeded indicates that the system as a whole is too sensitive. Therefore if the output of tube IB does not exceed a predetermined value, lamp 36 will be lighted when switch 34 is operated to close contacts 35. If the critical value is exceeded due to the fact that the condenser 5 is improperly adjusted the test lamp will not light because relay 4!) will have operated and opened its contacts 4!.

Thus if the test lamp lights but buzzer 30 does not sound, condenser 5 may be adjusted until the buzzer 30 is operated. If switch 34 will then light the test lamp it is evident that the system is properly adjusted to initiate an alarm when a person approaches the protected object within a given distance. Similarly, if the test lamp does not light on the first test, it is again evident that the circuit is not properly adjusted and the ca pacity 5 must be increased until the lamp does light. Here again if the buzzer and the lamp both operate the subscriber receives a positive indication that his system is operating properly. The test being completed, the subscriber throws switch 49 to the night position, which is shown in Fig. 1 of the drawing, whereby approach to the protected object will initiate an alarm at the central ofilce.

If the capacity alarm system is connected to a central station it is necessary to provide supervision of the system from the central station. This is accomplished by the use of relay 53 which is connected in series with the central station switch 45, battery and transmission line 43. Relay 5liis operable only on a comparatively high test voltage applied when switch 45 connects battery 46 to the relay. Operation of switch 45 causes relay 50 to close contact 5| thereby grounding condenser 32. This operation performs the same function as switch 34 and the operator receives an alarm if condenser 5 is properly adjusted and if the remainder of the system is in operative condition.

After the subscriber has tested his protective system and has adjusted it to its proper operating condition the approach of an unlawful intruder to the protected object or area will operate the system by balancing the bridge circuit in the same manner as described abovein connection with the circuit test involving the substitution of condenser 82; The bridge circuit becomes balanced thereby causing oscillations to cease. The potential across calls 6 and I de-' creases thereby decreasing the potential across inductance 28 to the point where the bias on the grid of tube l8 becomes zero. This allows the output of tube i8 to increase sufficiently to operate alarm relay 26 and actuate the alarm device 28 through contacts 21 and transmission line 48, since switch 49 is moved to its position completing the circuit to device 28 after all testing has ended.

It should be noted that the antennae 2 and 3 are connected between the bridge circuit, the protected object I and the oscillator tube ID. This particular feature of the invention provides a means of obtaining an alarm signal even though an attempt is made to defeat the system by cutting or otherwise tampering with the antenna leads. If an intruder approache the antenna he changes the characteristics of the bridge circuit to such an extent that it becomes balanced in the same manner as it would if he approached the protected object itself. If he cuts either antenna 2 or 3 the metallic connection between grid and ground is destroyed and tube l8 ceases to oscillate. If only one wir were used for the antenna connection, as is the usual practice, cutting the antenna would not necessarily cause an alarm, because the bridge would become more unbalanced than in the normal condition. But with this arrangement an alarm is always initiated in spite of attempts at tampering.

Fig. 2 of the drawing illustrates another type of capacity circuit wherein supervision of the antenna as well as the protected object itself is obtained also. This modification of the invention also illustrates a mean of obtaining much greater sensitivity to alarm conditions than is possible in previous types of capacity alarm circuits. In this modification of the invention the antenna 54 is disposed in the form of a loop around or within the object or area which it is desired to protect. The circuit consists of oscil lator tubes 80 and GI, together with their respective tuned-circuits which comprise condensers 62 and 63, together with the inductances 65 and 66. Normally, the variable condenser 62 is adjusted in such a manner that the frequency of oscillator tube 60 is almost equal to the frequency of tube BI and therefore when the outputs of tubes 88 and iii are fed into the input of mixer tube 88 its plate circuit contain the audio beat note of the two frequencies generated in tubes 88 and ii. The output voltage of the beat frequency which occurs in the output of tube 88 appears across the choke coil 12 which is connected between plate battery and the radio frequency filter consisting of coil 69 and condensers "I8 and II. This filter eliminates substantially all of any other frequency components which may appear in the output of tube 68. The coil 12 is coupled to the rectifier 75 by a network comprising the coupling condenser 13 and resistor 14 which is frequency discriminating over the range of audio frequencies normally obtained and thus as the constant potential energy available across coil 12 varies in frequency the voltage across resistor 14 varies in proportion thereto. Varying frequency being changed to varying amplitude, the diode rectifier 15 applies a rectified voltage across the resistor II and condenser I8. Thi voltage is applied to tube 88 as a bias which normally prevents emission in this tube and in turn prevents energization of the alarm relay 82.- Relay 82 controls the alarm 84 through the contact 83 and battery 85.

When a person approaches the antenna the capacity to ground is changed and the frequency of oscillator 88 decreases thus changing the beat note between the frequencies generated by tubes 68 and SI. Thus if the original frequency of oscillator 88 was higher than the frequency of tube 6| the beat note decreases in frequency and a lower frequency audio voltage appears across the coil 12. A lower frequency audio voltage also appears across the series combination of condenser and resistor 14 and because of the discrimination by condenser 13 against the lower frequency, a smaller audio voltage is applied across resistor 14. Therefore a similar rectified voltage is impressed on the grid of tube 80 allowing the plate current of this tube to increase and energizethe alarm relay 82 causing contacts 83 to close and the alarm 84 to operate.

As described above the two oscillator tubes 68 and iii are adjusted to be almost equal in respect to their output frequencies but this arrangement does not provide the ideal in sensitivity and therefore it is desirable to increase the sensitiviiy of the system as a whole. In this system as so far described the beat note changes exactly the same number of cycles per second as does the frequency of tube 68 and it is this change in frequency that determines the sensitivity of the system. Obviously if the antenna capacity is large a small change in it will cause only a small change in the frequency of tube 68, especially if this frequency is comparatively low, and the beat note will likewise change by only a few cycles per second.

Therefore, in accordance with this invention, the frequency of the tube Si is made equal to a higher frequency than that of tube 68 and has an approximately integral relationship to the frequency of tube 60. In other words it is n times the frequency of tube 80, where n is any integer. Under this condition, the frequency of tube 60 can no longer beat with that of tube iii to prouuce an audio beat note, but the nth harmonic of the first frequency can, and does, beat with the fundamental of the second frequency to produce the required audio beat note.

It is evident that when a variation of the antenna capacity causes the frequency of tube 60 to change 1 cycles per second, the nth harmonic will change n1 cycles, and the audio beat note in the plate circuit of tube 68 will likewise change 1f cycles. Thus the sensitivity of the system is raised n times by adjusting the frequency of tube iii to be n times the frequency of tub 60, and then using the nth harmonic of the second frequency to beat with the fundamental of the first frequency.

It should also be noted that it is also possible in this circuit as in that illustrated in Fig. 1 of the drawing to supervise the antenna in the same manner and with the same advantageous results. It is also possible to test the circuit shown in Fig. 2 in the same manner as illustrated in Fig. 1 since condenser 62 in Fig. 2 can be grounded by a switching device such as 34 shown in Fig. l to simulate a person approaching the antenna. Obviously the remainder of the test circuit can also be applied to the circuit shown in Fig. 2 and can be operated in the same fashion.

It is obvious irom the above description that this invention provides three very important features, namely: testing circuit and (3) greater sensitivity by making use frequency multiplication without unduly complicating the circuit. Further modifications and advantages of this system will be apparent to those skilled in the art and therefore it is desired that this invention shall not be limited to the modifications described and illustrated herein but shall be limited only to the required by the prior art and the appended claims.

We claim:

1. In a signaling system, a protected object having a capacity to ground which varies on the approach of a foreign body, a bridge circuit which includes said capacity and which is normally unbalanced but becomes balanced on the approach of a foreign body toward the protected object, a normally oscillating vacuum tube so controlled by the bridge circuit that unbalance of th bridge maintains oscillations while balance of the bridge stops oscillations, a protective antenna for connecting the bridge circuit and the protected object to the vacuum tube with the protected object between the bridge circuit and the vacuum tube whereby tampering with the protected object or with the antenna varies the capacity within the bridge and causes said vacuum tube to cease oscillating and an alann connected in operative relation to the oscillating circuit whereby cessation of oscillations initiates an alarm.

2. In a signaling system, a protected object having a capacity to ground which varies on the approach of a foreign body, a bridge circuit controlled by said capacity' and normally unbalanced but adapted to become balanced on the approach of a foreign body toward the protected object, a normally oscillating vacuum tube so controlled by the bridge circuit that unbalance of the bridge maintains oscillations while balance of the bridge stops oscillations, an antenna for connecting the bridge circuit and the protected object to the vacuum tube with the protected object between the bridge circuit and the vacuum tube whereby tampering with the protected object or with the antenna balances the bridge and causes oscillations to cease and an alarm connected in operative relation to the oscillating circuit whereby cessation of the oscillations initiates an alarm.

3. In a signaling system, a protected object having a capacity to ground which varies on the approach of a foreign body, a bridge circuit con trolled by the capacity, .an oscillating circuit so controlled by the bridge circuit that unbalance of the bridge maintains oscillations while balance of the bridge stops oscillations, an antenna for connecting the protected object and the bridge circuit to the oscillating circuit with the protected object between the bridge circuit and the oscillating circuit whereby tampering with the protected object or with the antenna unbalances the bridge and causes oscillations to cease and an alarm connected in operative relation to the oscillating circuit whereby cessation of oscillations initiates an alarm.

4. In a signaling system, a protected object having a capacity to ground which varies with the approach of foreign bodies, a signaling circuit for protecting said object comprising an oscillatory circuit and control means responsive to changes in the capacity to ground of the protected object for controlling the oscillatory circuit, an antenna connecting the oscillatory cir cuit to the control means with the protected ob- (1) antenna supervision, (2) a extent ject connected between the oscillatory circuit and the control means whereby tampering with said antenna causes a change in the output or the oscillatory circuit and a signal responsive to said change of output.

5. In a signaling system, a protected object having a capacity to ground which varies with the approach of foreign bodies, cooperating circuit means for measuring said capacity, an antenna in closed circuit relation between the respective circuit means and the protected object whereby tampering with said antenna or approach to the protected object aflects said means and an alarm connected in operative relation to the measuring means whereby changes in said capacity initiate an alarm.

6. Ina signaling system, a protected object, an alarm circuit for protecting said object comprising a source of oscillaitons, a balanceable circuit for controlling said source comprising the capacity to ground of the protected object and an adjustable capacity for adjusting the balanceable circuit, a first test circuit including a capacity equivalent to that of a person approaching the protected object and switch means for inserting this capacity in the balanceable circuit to create an alarm condition, and a second test circuit comprising means for measuring the output of the protective circuit and an indicating means connected thereto for indicating the condition wherein the output of the circuit is too great and the alarm circuit is too sensitive.

7. In a signaling system, a protected object, an alarm circuit for protecting said object comprising an oscillator tube and a bridge circuit for controlling said tube comprising the capacity to ground of the protected object and an adjustable capacity for adjusting the bridge circuit, a first circuit comprising a capacity equivalent to that of a person approaching the protected object and switch means for inserting this capacity in the bridge circuit to create an alarm condition and a second test circuit comprising a relay in the output of the protective circuit for measuring the output thereof and a lamp in series with the relay contacts for indicating the condition wherein the output of the circuit is too great and the alarm circuit is too sensitive.

8. In a signaling system, a signaling circuit for protecting an object comprising a balanceable circuit which becomes unbalanced when the electrical capacity of said object changes, means for adjusting the balanceable circuit, a first test circuit comprising a capacity equivalent to that of a person approaching the protected object, and switch means for inserting this capacity in the balanceable circuit to create a signal condition, and a second test circuit comprising means for measuring the output of the protective circuit and an indicating means connected thereto for indicating excessive sensitivity of the circuits.

9. In a signaling system, a signaling circuit for protecting objects comprising a circuit which responds to changes of electrical capacity, a first test circuit comprising elements for creating a signal condition and a second test circuit comprising elements for indicating excessive sensitivity of the signaling circuit.

10. In a signaling system, a signaling circuit for protecting an object, a circuit which responds to changes of electrical capacity and a test circuit comprising elements for creating an alarm condition and an indication of excessive sensitivity.

MAXWELL H. A. LINDSAY. KONSTANTIN WOLOSCHAK. 

